Summary
The actions of d-tubocurarine, lobeline and nicotine were studied at the end-plate region of frog sartorius muscle fibers bathed in Frog-Ringer solution containing 12 mM MgCl2. End-plate potentials (EPPs) evoked by stimulation of the sciatic nerve were monitored with KCl-filled microelectrodes using standard techniques. Quantal content (m) was calculated from the coefficient of variation of 110 repetitively evoked EPPs at 1 Hz.
Both d-tubocurarine (10−6 M) and lobeline (10−5 M and 2×10−5 M) decrease the amplitude of the evoked EPP without causing significant change in the resting potential of the end-plate membrane. Nicotine (10−5 M and 3×10−5 M) causes initial depolarization and subsequent repolarization of the end-plate membrane. During both these phases, the EPP amplitude progressively declines.
The decrease in EPP amplitude caused by d-tubocurarine and lobeline is due primarily to an interference with the postjunctional action of the transmittor as evidenced by a decrease in quantal size (q) without significant alteration in m. In contrast, the decrease in EPP amplitude caused by nicotine during its phase of maximum depolarization is accounted for by a decrease in both q and m. The depression of the EPP amplitude when the end-plate membrane has repolarized under the influence of nicotine is due to receptor desensitization.
These results indicate that at the frog neuromuscular junction the actions of lobeline more closely resemble those of d-tubocurarine rather than nicotine. The depression of m during nicotine-induced depolarization of the end-plate membrane may be related to a similar depolarization of the motor nerve ending.
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This research was supported by a grant from the AMA-ERF.
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Steinberg, M.I., Volle, R.L. A comparison of lobeline and nicotine at the frog neuromuscular junction. Naunyn-Schmiedeberg's Arch. Pharmacol. 272, 16–31 (1972). https://doi.org/10.1007/BF00498791
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DOI: https://doi.org/10.1007/BF00498791